One of the problems encountered in current horn-reflector antennas is the TM.sub.11 -mode "echo" signal generated in the input section of the horn due to the incident TE.sub.11 mode there. Thus, in the transmitting case, this undesired TM.sub.11 mode travels down through the waveguide feeding the horn until it encounters a waveguide transition at the lower end of that waveguide, and is then reflected back up through the waveguide feed and reconverted to the desired TE.sub.11 mode in the input section of the horn. This produces two transmitted TE.sub.11 mode signals which are not in phase with each other, thereby degrading the RPE (Radiation Pattern Envelope) and giving rise to a group delay problem which results in undesired "crosstalk" in the microwave signals.